Conventional hydrogen meters for use with liquid sodium employ a thin, probe-type nickel diffusion membrane which is immersed in the liquid metal. Hydrogen from the sodium diffuses through the membrane and enters an internal vacuum chamber where it is measured by an ion pump current. The current is a direct function of the hydrogen flux through the membrane and therefore an indication of the hydrogen concentration in the sodium.
For operation of a similar hydrogen flux device in liquid lithium, there are two fundamental problems to be overcome. First, the nickel membrane material typically employed in hydrogen meters for use with liquid sodium has a considerably higher solubility in lithium than in sodium at the temperature of interest. At 600.degree. C. the value in lithium is 0.01 mol percent nickel, whereas in sodium the corresponding solubility is 2.times.10.sup.-5 mol percent nickel. In a flowing loop system, such a high solubility would inevitably lead to significant loss of membrane material. Secondly, although dilute solutions of hydrogen in lithium obey Sievert's law, the equilibrium hydrogen pressures generated by such lithium solutions are as many as nine orders of magnitude lower than for corresponding sodium solutions. For this reason the flux produced across a conventional nickel membrane when subjected to a lithium solution would also be considerably lower and probably beyond the measuring capability of a conventional ion pump.